End-loaded piezoelectric unit



March 9, i948. H. M, BACH. I 2,437,660

END-LOADED PIEZOELECTRIC UNIT Filed April 29, 1944 '3 Sheets-'Sheet lv /4' 'WIM ATTORN EY March 9, 1948. H. M. BACH END-LOADED PIEZO-ELECT-RIC UNIT 3 Sheets-Sheet 2 Filed 'April 29, 1944 INVENTOR ATTORNEY l March 9, 1948. Hi M. BACH 2,437,650

END-LOADED PIEZO-ELECTRIC UNIT Filed April 29, 1944 s sheets-sheet .3

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l I l i Fifa Patented Mar. 9, 1948 END-LOADED PIEZOELECTRIC UNIT Henry M. Bach, Lawrence, N. Y., assignor to Premier Crystal Laboratories, Inc., New York, N. Y., a corporation of New York Application April 29, 1944, Serial No. 533,316

This invention relates to piezo-electric devices and more particularly to improved housings and assembly and contact arrangements for such devices.

A principal object of the invention is to Vprovide an improved'piezo-electric device of the end-loaded type.

Another object is to provide an end-loaded crystal device which is designed so as to facilitate assembly and loading, and which is dust-proof, moisture-proof and gas-tight.

.A feature of the invention relates to contacting arrangements, and to assembly fastening and retaining arrangements whereby the contact members also serve as assembly tools, and as a positive lock to prevent accidental loosening or turning of the contact prong fastening screws.

Another` feature relates to a T-shaped fastening bolt and cooperating perforated contact strip for facilitating location and tightening of the contact assembly.

Another feature relates to an improved housing having a non-metallic cover with intervening sealing gasket. The body of the housing to which the cover is attached is provided with special threaded inserts which are previously and permanently molded and anchored into the housing. As a result, the sealing gasket and cover plate can be easily aligned, and fastening of the cover plate does not result in twisting of the gasket.

Another feature relates to a crystal housing having a specially molded externally threaded insert for allo-wing a cover plate and rubber gasket to be easily aligned for fastening, vand avoiding the difficulties heretofore found to exist when the cover plate is fastened by removable screws threaded into internally tapped inserts. One of these difficulties was the existence of metal shavings, or dust which lodged at the bottom of the insert and prevented the proper tightening of the fastening screws.

A further feature relates to a sealed crystal housing in which a rubber or similar sealing gasket is employed in conjunction with a Bakelite strip or spacer. The gasket in addition to sealing the housing also acts as a retainer and cushion for the said spacer to prevent bodily displacement of the assembled unit within the housing.

Another feature of the invention relates to a holder which has the contacting system to the electrodes arranged symmetrically with respect to the two external contact prongs. As a result,

ternal contact strips are made narrow, their narrow limit being the width of the smallest crystal electrode that is used in the holder. Heretofore it has been necessary when the same size holder is to receive crystals of different sizes,

to provide contact strips which extend over substantially the entire width of the housing cavity so as to insure contact regardless of whether wide or narrow electrodes are used. The narrowing of the contact strips in accordance with the invention and their symmetrical location with respect to the center of the device, has the effect of limiting the effective shunt capacitance across the crystal which is of great importance in certain types of oscillator systems or circuits.

A further feature relates to a novel form of pressure contact spring for piezo-crystal units, which is especially designed so that the spring pressure on the crystal remains constant and substantially luniform over a very wide range of ambient temperatures e. g., -60 C. to +90 C.

A further feature relates to a novel coiled spring pressure arrangement wherein the coiled spring is especially designed so as to have uniform pressure or stiffness characteristics, and

` also as to electrical functioning characteristics so as to correlate the latter as a useful adjunct of an oscillator circuit in which the crystal unit is connected.

A stili further feature relates to the novel organization, arrangement and relative location of parts which cooperate to constitute an improved piezo-crystal device of the end-loaded type.

Other features and advantages not specically enumerated will be apparent after a consideration of the following detailed descriptions and the appended claims,

In the drawing, which shows certain preferred embodiments,

Fig. 1 is a plan view of the assembled device according to the invention.

Fig. 2 is a side elevation of Fig. l.

Fig. 3 is a top-end view of Fig. 1.

Fig. 4 is a sectional view of Fig. 3, taken along the line I-4 thereof.

Fig. 5 is a sectional view of Fig. 4, taken along the line 5-5 thereof.

Fig. 6 is a sectional view of Fig. 4, taken along the line 6-6 thereof.

Fig. 7 is a sectional view of Fig. 4, taken along the line 1--7 thereof.

Fig. 8 is a sectional View of Fig. 4, taken along the line 8 8 thereof.

Fig. 9 is an enlarged sectional view of the prong fastening and sealing arrangement.

Fig. 10 is an enlarged sectional view of the special arrangement for fastening the cover and gasket in place.

Fig. l1 is a perspective view of the contact strip and T-bolt fastening. Y

Fig. 12 is a view similar to modification.

Fig. 12a is a perspective View of thespacer'frame of Fig. 12.

Fig` 13 is a sectional View of Fig. 12, taken'along the line I 3-I3 thereof. Y

Fig. 75, but of Ya s above-mentioned diliculties are eliminated. Dirt and mold powder, chips, etc., cannot lodge in the insert, which can be any length necessary for'mechanical strength and it can be made more readily on automatic screw machines.

A further advantage of the threaded insert is that amaterial such as stainless steel may be Fig. 14 is a View similar to 12, but of a.`V .Y

still further modification.

Fig. l5 is a sectional View alongtheline i5-i5 thereof.- Fig. 16 isa plan View of one embodiment ofthe pressure spring according to the invention. f

Fig. 17 is a sideview of Fig. 16.`

Referring .to Figs, .1 to .5, the device Vcomprises an enclosing housing iof Bakelite or other suitable insul-atonmaterialwhich can be molded in one piece to `flat box-like shape.. having its top endop'en and its bottom endprovided with openings. 2.for the plug-in prongsll. Each-of the prongs has an integral polygonal flange or shoulder E. and a cylindrical end 'i which is internally threaded to receive the T-shaped fastening bolt 8 vas shownfmore clearly in Fig, 9. The bottom face YIB of the housing is circularly recessed around-each opening 2 to receive a sealing gasket ii Vof rubber, neoprene or the like.- Preferably, the under-cut .face .I0 is provided with .a circular ridge or .rib l2 which cooperates with gasket II to increase .the sealing efficiency. Likewise, the top open end of housing VI isclosed by a cover plate .I3 Vand an intervening .sealing gasket I4. Housing cover I3 -is preferably .molded of a lamin ated?.A phenolic .known as` BM97v25-Rogers board-which has much higher flexural strength than-usual mold materials, thus affording better seal. The inner edge of '.th'e top .surface isprovided with asealing rib for purposes similar toV ribs I2.Y and as described, in detail in of Fig. 14, taken (U. S..Patent No. 2,329,321. lThe underfacef. I 3

aroundholes 29 iscounterbored to enhance sealingby allowing the gasket toride up .against stud I9.

` For the purpose of facilitating the-assembly and fasteningf of the .cover and" gasket, `there are molded intothe top face of jhousngl, adjacent the corners, four .metal inserts. l5 (Figl) each comprising a-knurled cylindrical shank I6 terminatnsat its lower Yend in a circularlyundercut portion I 'i and .tip portion KI8 whereby the insert is rigidly anchored in` place during the molding operation. f The other end I 9 of. thejshank I6 is of reduced diameter` and passes.through corresponding openings in gasket' I4 and .cover I3. Cover I3 has circular recessesj toY freely receive the internally threaded split-,top nuts l2i. Preferably, althorgh not necessarily, themernbers I5 and 2l have left-handed threads and the lends ofnniembers I9 project sothat nuts 2'I cannot be turned by an ordinary screw-driver.

Y In the caseof the usual internallytapped-insert, its length is limited substantially to. a maX- imum of twice the diameter YVofV ,thetappedA hole. The inserts must be held by pins (tted :into .the tapped hole) in the mold and 'the pins will be Aused which is much stronger than brass. Brass must'be'used on internally tapped insert because of manufacturing impossibility of tapping the short blind internally tapped insert, Brass inserts strip in theratio of 1 to i0.V The stainless externally threaded insert is much stronger and does not turn oi or strip.

While any well-known form of crystal-electrode unit may be employed, it is preferred to employ a unit such as shown in detail in U. S. Patent No. 2,329,321, and comprising a crystal plate 22V (Fig. 4) which is sandwiched between a pair of metal electrodes 23, 2d. These `electrodes have their corners struck up or otherwise offset as by swedging soas to provide corner lands denfng an air gap 25 between the crystal plate and each of the electrodes. In order to make electric contact between the prongs and the respective electrodes, special contact strips 27 are provided one of which is shown in perspective in Fig. 1l. Each strip is formed of stainless steel and the lower end 25 is bent at an angle greater than 99 e. g., 91 and ts freely in the opening 2, With this arrangement, when part 25 is fastened against the prong shank, the part 2'I.is spring-pressed flat against the inside wall 28 of the housing (Fig. 7).- f The other contact strip is assembled in the opposite relation so that its vertical leg 2l is spring-pressed 'at against the inside wall 29 (Fig. 8). In order to fasten the contact strips in place each strip has an opening .39 through which freely passes the threaded shank` of the corresponding T-bolt 8. Each contact strip also has a rectangular opening 3'I to receive the laterally extended end 32 of the T- bolt (Fig. 11), thus preventing the latter from turning when the contact prong'is tightened. Furthermore, since the housing cavity is relatively narrow, it is not convenient to use screw drivers or the like to fasten ordinary slotted-head screws in place. With the arrangement as described, the bolts can be pre-assembled on their respective contact strips and can be easily lowered into place Within the cavity. Because of the relation'between the bolt head and opening SI (Fig. 11), as the bolt is tightened it causes the end 21 of the contact strip to be pressed flush against the housing wall. Y

It will be noted that the contact strips 21 are much narrower than the walls 28 and 29 and the strips are offset so that they are symmetrically -disposed with respect to the center line of the unit, notwithstanding that the two contact prongs are brought out at the same end of the unit and in the same plane. Heretofore, it has been necessary to employ contact plates which extended almost entirely across the width of the housing. Consequently, these prior' contactstrips introduced an undesirable electrostatic capacity in shunt to the crystal. With the contact arrangement as shown, the width of the contact strips 21 may be no greater than the width of the smallest crystal electrode that is used in the housing, it being understood that the housing is capable of accommodating crystals and electrodes of greatly varying widths, for example by using a special spacer frame to make up the difference in width between the crystal unit and the inside dimensions of the housing cavity (Figs. 12 and 13). The advantages of this definite control of the maximum inherent capacitance between the contact strips and the crystal will be explained in further detail in connection with Figs. 18, 19 and 20.

In order to prevent dislocation of the crystal electrode assembly, there is interposed between the electrode 24 and Contact strips 21 a specially designed spring plate 33 which is shown in detail in Figs. 16 and 17. This spring is fashioned out of spring stock, preferably of the alloy commercially known as Inconel, and the main body portion is substantially rhombic or diamond shaped and terminates in curved back bearing edges which are straight and parallel. This spring is, in accordance with one of the features of the invention, designed to meet two conditions which are of peculiar importance in the piezocrystal art. (1) Its force on the crystal must remain constant over a very wide range of ambient temperature e. g., 60 C. to +90 C. (2) It must be able to withstand excessive shocks e. g., those encountered in drop testing of the entire device, without exceeding the elastic limit. I have found that by giving the spring plate the shape shown and by properly proportioning its dimensions, these objects are simultaneously achieved. The spring possesses minimum stiffness or K where K equals dF/dI-I, dF/dH being the rate of change of spring force with height H (Fig. 17), for a given length L and Width D (Fig. 16). In order to prevent the spring exceeding its elastic limit, its eifective height H cannot be reduced below a certain minimum and for this reason the length L is slightly less than the width of the housing cavity. This allows the spring to act as a uniform pressure device over the desired ambient temperature range. In addition when the assembly as a whole is subjected to a shock which tends to reduce the height H below a value that would exceed the elastic limit, the length L increases sufciently to engage the side walls of the cavity. This eiectively limits the reduction in height for excessive shocks. One spring that was found to possess the desired uniform spring pressure and which was protected against excessive shock had the following dimensions when made of Inconel. Thickness is .007 inch, L is .9()0 inch, W is .785 inch and stiffness is 29.52 lbs/inch. Such a spring in normal service conditions will operate over a temperature range of 68 C. to +90 C. with a restoring force (for an average fixed height H), which does not change from its normal value (i. e. value at 25 C.10 C.) when subjected to the said variation in ambient temperature. The assembly is spaced from the bottom of the housing by a Bakelite strip 35, and a similar Bakelite strip 36 engages the top oi the assembly. When the cover plate is fastened in place it causes the rubber gasket adjacent the sealing rib to be slightly displaced downwardly to provide a retheir respective sealing washers are iitted into the opening 2. The T-bolts previously assembled in their contact strips are lowered into the cavity so that the bolts 8 are in registry with the prongs. The latter are then turned to fasten the contact strip rigidly in place. Then the Bakelite strip 35 is dropped into place. The parts 23, 2d, 25 and 3S, are previously assembled in a special open-ended jig, which can be aligned with the housing cavity and the assembled parts can then be ejected from the jig directly into the said cavity. The Bakelite strip 35 is then placed in position, and then the gasket and cover plate are positioned, whereupon nuts 2l are tightened to seal the cavity.

1n the event that the crystal-electrode unit is smaller than the housing cavity, it may be preassembled inside a rectangular mask or frame il@ (Figs. 12 and 13) of Bakelite which closely fits the cavity. The end walls of this frame may be cut away (Fig. i2) to facilitate removal of the assembly from the holder.

Instead of using a crystal-electrode unit wherein the crystal is clamped between electrodes asin Figs. l to 12, the electrodes may be separated along their opposite lateral edges by glass spacers lll, 42 (Figs. 14 and l5), in which event the thickness of the crystal plate is approximately .G01 to .0003 inch less than the thickness of the glass spacers lll, d2. Spacers di, d'2, may be of glass, ceramic, quartz or other similar insulating material.

When it is desired to seal the holder with a gas content therein, at atmospheric pressure, the device is assembled as above described and the top cover plate is fastened tightly. Then one of the contact plugs is loosened. The unit can then be flushed under a bell jar and then lled with the desired gas or mixture of gases at atmospheric or slightly higher than atmospheric pressure. The device is then removed from the bell jar and the loosened plug is tightened to seal the gas inside the housing.

While certain specic embodiments have been disclosed, it will be understood that various changes and modications may be made without departing from the spirit and scope of the inven tion.

What I claim is:

' 1. A piezo-electric device comprising a housing of insulation material having one end open for loading a crystal-electrode unit therein, a plurality of metal inserts molded directly into andY anchored in the body of the housing adjacent the open end, a sealing gasket and a cover plate of insulation for sealing said end, said metal insert having an external threaded portion adapted to pass through corresponding openings in the gasket and cover, and threaded nuts for said exu ternal threaded. portions of the inserts for fastening said cover and gasket without subjecting the gasket to twisting.

2. A piezo-electric device comprising a housing of insulation material having one end open for loading the crystal-electrode unit therein, a pair of contact prongs, a corresponding pair of metal contact strips one for each electrode, and a pair of T-bolts for fastening the contact strips internally to respective prongs, the fastening being effected by turning the prongs externally.

3. A piezo-electric device according to claim 2 7. inV which each Vcontact strip' has its lower end bent at right angles and provided with an opening for the passage of the T-bclt, the unbent portion of the strip having an opening to receive the end of the bolt head to prevent the bolt turning when the prongs are externally turned for fastening.

4. A piezo-electric device comprising a ila-ttened box-like housing of insulation material, a pair of openings at one end of the housing to receive the internally threaded shanks ofV a pair of contact prongs, a pair of L-shaped contact strips within the housing, and a pair of T-bolts one for each contact strip for fastening it to a corresponding prong, the contact strips having their main body portions resting iiat against opposite interior walls of the housing.

5. A piezo-electric device according to claim 4 in which the external surface of the housing surrounding said pair of openings has circular sealing ribs, and a sealing washer is located between each rib and an integral flange on the correspending contact prong.

6. A contact plate for end-loaded piezo-crystal housings comprising a metal strip having its lower end bent substantially at right angles, an opening in said bent end to receive a threaded T-bolt, and another opening in the unbent portion to receive the head of the T-bolt whereby said contact plate can be used as a holding tool to locate the bolt with respect to an internally threaded prong, the head of kthe bolt` cooperating with said other opening to allow downward pressure to be exerted on the contact plate during assembly without dislodging the bolt.

7. A piezo-electric device comprising a flattened box-like housing of insulation material, a pair cip-ronge tted into one end of the housing, each of the prongs having an internally threaded shank to receivel a T-bolt interiorly of the housing, a pair of contact plates having right-angled vportions for clamping between the head of the T-bolt and the said threaded Shanks, a crystalelectrode unit within the housing cavity. 'a rigid spacer strip at the end of the unit and substantially flush with the top open end of the housing, a sealing gasket and cover plate for sealing said open end, and means for fastening said gasket and cover plate comprising metal inserts molded into the housing with upwardly projecting threaded shanks for passage through corresponding openings in the gasket and cover plate, said gasket engaging said rigid spacer strip and acting as a resilient -cushion therefor.

8. A piezo-crystal device comprising a housing, a crystal and electrode unit within the housing, and a pressure spring in the form of a bowed plate the main body portion of which is substantially rhomboidal, said spring having smoothly rounded parallel bearing edges which rest against the wall of the housing cavity while the bowed portion presses against one of the crystal electrodes.

9. A piezo-electric device according to claim 2 in which said -contact strip is in the form of a metal plate having one section substantially L-shaped in a plane parallel to the housing wall and another section extending substantially at right angles to the rst section, the last section being assembled with respect to said T-bolt so that tightening of said holt causes the first section to be pressed fiat against said housing wall.

HENRY M, BACH.

REFERENCES GITED The following references are of record in the file of this patent:

UNTED STATES PATENTS Number Name Date 1,434,856 Stenson Nov, 7, 1922 2,256,932 Wolfskill Sept. 23, 1941 2,329,321 Bach Sept. 14, 1943 2,361,343 Woodmark Oct. 24, 1944 2,392,690 Perkins et al. Jan. 8, 1946 

